This invention relates to a phosphate supported chromium catalyst for olefin polymerization.
Supported chromium oxide catalysts can be used to prepare olefin polymers in a hydrocarbon solution to give a product having excellent characteristics from many standpoints. Silica supported chromium oxide catalysts can also be used to prepare olefin polymers in a slurry system wherein the polymer is produced in the form of small particles of solid material suspended in a diluent. This process, frequently referred to as a particle-form process, has the advantage of being less complex. However, certain control operations which are easily carried out in the solution process are considerably more difficult in the particle-form process. For instance, in the solution process, control of the molecular weight can be effected by changing the temperature with lower molecular weight (higher melt flow) being obtained at the higher temperature. However, in the slurry process, this technique is inherently limited since any efforts to increase the melt flow to any appreciable extent by increasing temperature would cause the polymer to go into solution and thus destroy the slurry or particle-form process. It is known to extend the range of melt flow capability of a given catalyst through the use of oxygen. However, it has not heretofore been commercially feasible to produce a complete spectrum of polymers so far as melt flow is concerned in a slurry system with a single chromium catalyst system. Also it is frequently desired to have a polymer with greater shear response than is normally obtained in the slurry or particle-form process.
While it has long been known to use metal phosphates as supports for chromium olefin polymerization catalysts, such supports have never achieved commercial success. Thus chromium catalysts are generally supported on silica-containing bases similar to those used decades ago when the chromium catalyzed polymerization of olefin polymers first became commercial.